1. Field of the Invention
The present invention relates to a printer system that prints a multi-tone image with a head, which enables at least two different types of dots having different density evaluation values, which represent densities per unit area and vary in a stepwise manner, to be created with respect to at least one hue. The present invention also pertains to a method of printing as well as a recording medium for implementing the method.
2. Description of the Background
Color printers that eject multiple colors of inks from a head (ink jet printers) are widely used as an output device of a computer, which prints images processed by the computer in a multi-color, multi-tone manner. In the ink jet printer, the tone corresponding to input tone data is expressed by regulating the creation ratio of dots in each area.
The error diffusion method is one of the methods that regulate creation of a dot in each pixel based on the above idea. The error diffusion method diffuses an error, which occurs in each target pixel due to a discrepancy between the density to be expressed in the pixel by the input tone data and the density expressed by a dot actually created in the pixel, into peripheral non-processed pixels in the vicinity of the target pixel. Such error diffusion results in regulating creation of dots, in order to minimize the density error in a resulting image as a whole. Correction data, which is obtained by making the diffused errors reflect on the input tone data, i.e. errors are distributed into adjacent unprocessed pixels, is used to determine creation or non-creation of a dot in each pixel.
A variety of techniques have recently been proposed to enrich the tone expression in ink jet printers. One of such techniques is a printer using inks of different densities and a corresponding method of printing (for example, JAPANESE PATENT APPLICATION No. 8-209232). This technique provides a higher-density ink and a lower-density ink with respect to an identical hue and regulates ejection of these inks, in order to attain the printing with excellent tone expression.
Another proposed technique for attaining the multi-tone expression is a printer that creates two different types of dots having different ink densities and different dot diameters and thereby varies the density per unit area in multiple stages (for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 59-201864). In this printer, one pixel consists of four dots. The technique changes the frequency of the appearance of the dots having the higher density and the dots having the lower density in each pixel, so as to enable an image to be printed in multiple densities.
These proposed techniques do not carry out the two-valuing process, that is, the simple on-off determination of dots, with respect to each hue, but implement the three-valuing or another multi-valuing process with various types of dots having the different densities or different dot diameters, in order to attain the multi-tone expression.
The error diffusion method significantly reduces the density error in a resulting image as a whole and improves the picture quality of the resulting image, compared with other methods that regulate creation of dots. The error diffusion method is accordingly advantageous for the two-valuing process. It has been proved, however, that the three-valuing process according to the error diffusion method can not regulate the creation ratio of dots to a desired value. An example of such results is shown in the graph of FIG. 14.
The graph of FIG. 14 shows a creation ratio plotted against the input tone value in the range of 0 to 255. The creation ratio of the greater diameter dots (hereinafter referred to as the large dots) to the total number of dots including also the smaller diameter dots (hereinafter referred to as the small dots) was determined by extending the error diffusion method to the three-valuing process, where threshold values were set to fixed values ‘64’ and ‘94’, a density evaluation value of the small dot to 128, and a density evaluation value of the large dot to 255. In the event that correction data, which was obtained by making the diffused errors reflect on the input tone value, was smaller than the value ‘64’, it was determined that no dot was to be created. In the event that the correction data was not smaller than the value ‘64’ but was smaller than the value ‘94’, it was determined that a small dot was to be created. In the event that the correction data was not smaller than the value ‘94’, it was determined that a large dot was to be created. After the determination of creation or non-creation of a dot in a certain target pixel, the process diffused a density error, that is, a difference between the density evaluation value of the dot actually created in the pixel and the density to be expressed in the pixel by the input tone value, into peripheral pixels and carried out the processing for a next target pixel. The proportion of the error diffusion into the peripheral pixels is not essential, and the description is omitted here.
As clearly shown in FIG. 14, the creation ratio of the large dots obtained as a result of the above process generally shows an increasing tendency according to the input tone value, but does not change smoothly and has some extreme points. At some of these extreme points, the creation ratio of the large dots abruptly changes (for example, points p and q in FIG. 14). Even if the density corresponding to the input tone value is expressed in a resulting image as a whole, the expression with a small number of large dots and the expression with a large number of small dots have the different visual effects. The large dots generally have a higher visual recognition then the small dots. The frequent appearance of the large dots thus undesirably increases the harshness of the resulting image. At specific input tone values where the creation ratio of the large dots significantly changes, for example, the points p and q in FIG. 14, the resulting picture quality is different from the picture quality in a peripheral image area. This effect may result in the occurrence of a quasi contour.